Diesel engines provide lower emissions and increased fuel economy as compared to gasoline engines; however, environmental and health hazards are posed by diesel exhaust emissions. Diesel particulate filters control particulate emissions from diesel-powered equipment such as trucks, buses, diesel electric locomotives and generators. Cordierite is a preferred material for the manufacture of diesel particulate filters and other high temperature bodies such as catalytic converters, NOx adsorber substrates, catalyst substrates, honeycomb monoliths and flow filter bodies because it is relatively low cost. Cordierite-based composite materials also have a low coefficient of thermal expansion to allow a part to withstand high and variable temperatures.
Diesel particulate filters control diesel particulate emissions by physically trapping soot particles in their structure. Wall flow monoliths such as cordierite filters allow exhaust gasses to flow through the porous ceramic walls, while any particulate is collected on the upstream side of the wall. The filter may be cleaned by auto-regeneration, during which the temperature of the exhaust gas is high enough to ignite any particulate and thereby reduce the backpressure of the diesel particulate filter. The surface of the upstream wall may contain a catalyst wash coat of platinum (Pt), iron (Fe), strontium (Sr) or rare earth elements such as cerium (Ce) to lower the temperature required for auto-regeneration of the filter.
One preferred material for the manufacture of high temperature components is cordierite (Mg2Al4Si5O18), a magnesium aluminum silicate that often includes low levels of iron or other impurities. Cordierite ceramics have a low coefficient of thermal expansion (CTE), high strength and are resistant to thermal shock. Cordierite materials are typically manufactured by mixing a raw batch including talc, alumina, aluminum hydroxide, kaolin and silica. The batch is then blended with a binder such as methylcellulose and lubricants such as sodium stearate to form a plastic mixture that is formed into a green body and sintered. U.S. Pat. No. 6,864,198 (hereby incorporated by reference in its entirety), assigned to Corning Incorporated, discloses a method of forming cordierite honeycomb structures. The '198 patent states “[t]he green bodies are dried and then fired at a sufficient temperature and for a sufficient time to form the final product structure. The firing is preferably done by heating to a maximum temperature of about 1405 to 1415° C. over a time period of 50 to 300 hours.”
Geopolymers are described in U.S. Pat. Nos. 4,349,386; 4,472,199; and 5,342,595, herein incorporated by reference in their entirety. The geopolymers described are ceramic-like materials with framework structures that can be synthesized at low or ambient temperatures. Geopolymers are typically inorganic materials produced by the polymerization and solidification of mixtures of “active” metakaolin and silica under highly alkaline conditions. The geopolymer materials disclosed in these patents are suggested for products such as building materials and thermal insulation. Geopolymer materials have also been studied for use in bonded abrasive products and for the immobilization of intermediate-level nuclear and toxic wastes.
U.S. Pat. No. 5,244,726, herein incorporated by reference in its entirety, discloses an alkali metal silicate-based matrix that includes inorganic particulates, organic particulates, or a mixture of inorganic and organic particulates which is produced at ambient temperature by activating the silicates of an aqueous, air-entrained gel containing matrix-forming silicate, particulates, fly ash, surfactants, and a pH-lowering and buffering agent. The '726 patent states that particulates should be present in amounts between 30 and 40 volume percent, and particulates such as polystyrene beads for foaming and wollastonite for strengthening the composite material are specifically mentioned.